Method for making free-flowing coated rubber pellets

ABSTRACT

Free flowing coated pellets are prepared by blending a first polymer with a second polymer wherein the second polymer is a polymer which has a crystalline or semicrystalline melting point which is at least 10° C. above the melting or softening point of the first polymer, the two polymers being insoluble in one another in the melt state; (2) intimately blending the two polymers; and (3) extruding the blend through a die having an outlet die face which is maintained at a substantially lower temperature than the extruder melt temperature. 
     In its preferred embodiment the first polymer is an elastomer and the second polymer comprises at least one crystalline or semi-crystalline plastic polymer. A typical composition comprises an EMP or EPDM rubber and a mixture of polyethylene and a crystalline copolymer of ethylene and propylene.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division, of application Ser. No. 696,726, filed Jan. 31.1985, now U.S. Pat. No. 4,622,193, granted Nov. 11, 1986, which is acontinuation-in-part of U.S. Patent Application Ser. No. 621,179 filedJune 15, 1984, now abandoned.

BACKGROUND OF THE INVENTION

Many elastomers are tacky or exhibit cold flow in their green or uncuredstate. As a consequence, these materials cannot be transported in bulkas free flowing pellets but must be shipped in bales. This practicerequires that the ultimate elastomer processor must be equipped to cutup or mill the bales. The necessary equipment is generally large scale,expensive equipment. Additionally, the bales cannot be readilypreblended with other materials. The necessity for baling results inhigh handling and shipping costs. In order to facilitate handling andprocessing of elastomers, it has been considered desirable to produceelastomer pellets. Generally, however, elastomer pellets exhibit"blocking" or cold flow characteristics which result in solidificationinto a solid mass after a short storage time, especially at warmtemperatures.

Numerous attempts have been made to formulate elastomeric pellets whichwill remain free flowing until they are to be processed. Dusting theelastomeric pellets with inorganic materials, e.g., clay, talc, etc.,has been found to extend the time over which the pellets are freeflowing. Improved results have been achieved by dusting a coating withselected organic materials such as hydrocarbon waxes (British Pat. No.901,664) or powdered polyethylenes and polypropylenes (British Pat. No.928,120). However, because of the discontinuity of the dust coat, thecoated pellets eventually flow together to form a solid mass.

By blending the elastomer wiht a crystalline type polymer such aspolethylene, polypropylene or copolymers of ethylene and propylene, ithas been possible to produce free flowing elastomer containing pellets.However, the elastomer content of the pellet must be less than about65%. The product is, of course, not suitable for use in all elastomerapplications.

Another coating approach to the problem has been the coating ofelastomer pellets with emulsions containing a tack free coatingmaterial. Coating is accomplished either by dipping pellets into theemulation or spraying the emulsion onto the pellets. In either case theemulsion coating must be dried, and where the emulsion contains asolvent the solvent must be recovered. Drying and solvent recoveryrequirements result in increased costs.

Melt-coating methods for producing free-flowing elastomer pellets havealso been suggested. According to U.S. Pat. No. 3,669,772 to Bishop,coating can be accomplished by using a die, similar to wire coating die,into which a strand of rubber to be coated is fed simultaneous with meltcoating material. A continuous melt coated strand of rubber issues fromthe extrusion die outlet, is cooled in a liquid cooling bath, and issubsequently pelletized. This melt-coating method not only addssignificantly to rubber manufacturing costs, but has limitations fromthe standpoint of efficiently producing large quantities of coatedpellets.

Pellets of rubber have been coated with various coating materials byheating the rubber pellet to a temperature which is higher than themelting point of the coating materials, and then contacting the heatedpellet with the coating material which is preferably in the form of afine powder. The heated pellet fluxes the coating material on thesurface of the pellet to form a substantially continuous coating. Thehot coated pellet is then cooled.

A study of bicomponent mixtures has shown that upon extrusion of themixtures, stratification will occur. See Soulborn, J. H. and Ballman, R.L.; "Stratified Bicomponent Flow of Polymer Melts in a Tube", AppliedPolymer Science, No. 20, 175-189 (1973). The authors attributestratification to differences in the melt viscosity of the components.

What is required by the art is a commercially viable method of producinga free flowing rubber pellet which contains a major portion of rubber.To be commercially viable, the process for producing the free flowingpellets must be readily adapted to conventional rubber pelletizingtechniques.

SUMMARY OF THE INVENTION

It has surprisingly been found that a rubber pellet compositioncomprising an elastomer and plastic insoluble in the elastomer can becaused to coat itself with a plastic skin by control of composition,extrusion conditions and die temperature. The elastomer-plastic blend isextruded at a temperature above the melting point of the plastic, andpelletized as it exits from a die, the die having a temperature gradientacross the die from inlet to outlet, the gradient being such that thedie outlet temperature is substantially lower than the extrusion melttemperature (die inlet temperature). The resultant product is a pelletcoated with a skin of plastic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A--Scanning Electron Microscope (SEM) micrograph of strand-cutpellet sections microtomed at liquid nitrogen temperatures.

FIG. 1B--SEM micrograph of under-water pelletized pellet sectionsmicrotomed at liquid nitrogen temperature.

FIG. 2A--SEM of surface of strand-cut pellet section microtomed atliquid nitrogen temperatures showing rubber extracted from the surface.

FIG. 2B--SEM of surface of under-water pelletized pellet sectionmicrotomed at liquid nitrogen temperatures showing continuousthermoplastic skin with no rubber extracted.

DETAILED DESCRIPTION

This invention relates to a method for preparating a free flowingelastomer pellet. More particularly, it relates to a method forpreparing an elastomer pellet which is free flowing by virtue of thefact that it is encased in a skin comprising a solid plastic material.

In the practice of this invention an elastomer is blended with a semicrystalline or crystalline plastic material which has a melting point ofat least 10° C. higher than the softening point of the elastomer,preferably at least about 15° C. than the softening point of theelastomer, preferably at least 30° C., more preferably at least 40° C.The elastomer/plastic blend is then extruded through a die in which thedie outlet is maintained at least 10° C. below the melting point of theplastic in order to develop a temperature gradient across the die fromdie inlet to die outlet, preferably at least 20° C., more preferably atleast about 30° C. below the melting point of the plastic.

Not wishing to be bound by theory, it is believed that as the melttemperature is reduced across the die, the difference in viscositybetween the elastomer and the plastic is increaseed thereby causingstratification in a manner so as to cuase the plastic to be concentratedalong the surface of the die orifice while the central core becomeselastomer rich. Shear plays an important part in the stratificationprocess as does the wall effect because of their effect on the velocityprofile of the two components of the melt, and hence, the compositiondifferences throughout the melt exiting the die. The melting point ofthe polymer is a function of shear and pressure, and is higher in thedynamic system of an extruder die than the static melting point of thepolymer. As used in the specification and claims, "melting point" willmean the normal static melting point or softening point of the polymer.

The elastomer/plastic polymer composition is extruded through amulti-orificed strand die in the aforedescribed manner and pelletizedeither by use of a strand pelletizer or by using a die face pelletizer.In one embodiment a conventional strand die is modified by having thedie outlet plate cored so that it can be water cooled. In a preferredembodiment the die cooling is accomplished by using an underwaterpelletizer. Typical of these underwater pelletizers is the miniunderwater pelletizer (MUP) manufactured by Gala Industries, Inc., EagleRock, VA.

Since the stratification proces by which a pellet coated with a skin ofplastic is formed requires a finite time the L/D ratio of the die outletholes is an important criterion in carrying out the process of thisinvention. The L/D ratio can be about 2 to about 20, preferably about2.5 to about 12, more preferably about 3 to about 10, e.g., about 3.5 toabout 8. The die outlets through which strands of elastomer/plasticblends are extruded can be converging tubular outlets which have alarger diameter inlet than outlet. In that event, the L/D ratio is basedon an average outlet diameter over the length of the channel.

The length of the outlet channel can be about 1 inch to about 4 inches,preferably about 1.5 to about 3.5 inches, more preferably about 2.0 toabout 3.0 inches, e.g., about 2.5 inches. The diameter of the die outletorifice can be about 0.05 to about 0.200 inches, preferably about 0.075to about 0.150 inches, e.g., about 0.125 inches.

A critical parameter in carrying out the process of this invention isthe temperature gradient across the die from the inlet to the outlet.While no particular temperature gradient is required, at some pointwithin the die the melt temperature must be reduced to a temperaturewhich is preferably at about the melting point of the plastic in orderto insure that there is a significant difference between the viscosityof the plastic melt and the viscosity of the elastomer melt. It is notessential that the melt temperature of the composition be below that ofthe plastic melt point. In a preferred embodiment, however, the melttemperature of the composition is reduced to a temperature which isbelow the melting point of the plastic component. In the preferredmethod of carrying out the process of this invention, an underwaterpelletizer is used and the temperature gradient across the die iscreated by cooling the face of the die.

The maximum temperature differential across the die is achieved byoperating at or about the plugging temperature of the system. The"plugging temperature" is that temperature at which some of the dieoutlet orifices begin to be plugged by solidified polymer. Some pluggingof a multi-orifice die can be tolerated up to the point where flow rateis decreased below economical rates. Generally, the outer outlet holesin the die will plug first. A multi-orifice die will have twenty or moreoutlet holes, e.g., 50-100. It is possible to operate the die at theplugging temperature with as much as about 20-30% of the holes plugged.

The plugging temperature is determined by gradually cooling the die ordie face to the point where outlet hole plugging begins to ccur.Operation at the plugging temperature achieves the maximumstratification and plastic skin development in the elastomer pellet.

Where an underwater pelletizer is used, the cooling water temperaturewill be about 20° C. to about 50° C. The "extruder melt temperature"(the die inlet melt temperature) will be about 160° C. to about 250° C.and will depend on the elastomer and plastic selected. The appropriateextruder melt temperature for various plastic/elastomer compositions isknown to those skilled in the extrusion art.

The process of this invention is particularly suited to those elastomerswhich are tacky in their solid state or exhibit cold flow. Illustrative,non-limiting examples of the elastomers to which this invention may beapplied are high molecular weight elastomers having a Tg of less than 0°C., e.g. ethylene-propylene rubber (EPR), terpolymers of ethylene,propylene and a non-conjugated diene (EPDM), natural rubber,polyisobutylene, butyl rubber, halogenated butyl rubber,arylonitrile-butadiene rubber (NBR) and styrene butadiene rubber (SBR)or mixtures thereof.

The plastics which may be utilized in the practice of this inventionhave a crystalline melting piont of at least 70° C. Illustrative ofthose plastic polymers are high density polyethylene (HDPE), low densitypolyethylene (LDPE), polypropylene (PP), linear low density polyethyleneLLDPE, syndiotactic polybutadiene resin (SBD), polybutene-1 andcrystalline copolymers of ethylene and other alphaolefins. The plasticand elastomer must be insoluble in one another in the melt state.

The elastomer-plastic polymer composition of this invention can compriseabout 15 to about 90 weight percent plastic polymer, e.g., about 20 toabout 80 weight percent. Where the product desired is an elastomericproduct the plastic polymer comprises about 15 to about 35 weightpercent of the composition; preferably about 15 to about 30 weightpercent, most preferably about 20 to about 28 weight percent, e.g.,about 25 wt.%.

The plastic and polymer may be blended in any conventional manner andfed to an extruder. For example, an elastomer bale can be shredded andblended with plastic polymer powder in a ribbon blender and subsequentlyfed to an extruder. Preferably a mixing extruder, e.g., twin screwextruder is used for the extrusion to insure complete mixing of theelastomer and plastic. The mixture is extruded out of a conventionalmulti-orificed die in which the die face is maintained at a temperatureof at least about 10° C. below the melting point of the plastic polymer.Preferably the die face is maintained at a temperature at least about10° C. below the melting point of the plastic polymer; more preferablyat least about 20° C.; most preferably at least about 30° C. below themelting point of the plastic. Of course, in view of the high melttemperature of the polymers the entire die plate cannot be maintained ata single temperature, and there will be a temperature gradient acrossthe die from its internal inlet surface to its outer face at the outletof the die.

To demonstrate the effectiveness of the instant invention, anelastomer-plastic polymer composition having the formulation shown inTable I was extruded through a conventional multi-orificed strand dieand cooled by passing the polymer strands through a water bath.Subsequently, the strands were pelletized. Additionally the sameformulation was pelletized using an underwater pelletizer.

The underwater cut pellets had a plastic skin, a lower coefficient offriction and were more free flowing than the conventional strandpelletized material. Table II compares the coefficient of friction ofthe two products, and Table III shows the pressure/strength ratio forthe compositions. The pressure/strength ratio is the ratio of theconsolidation pressure to yield strength under the shear required tocreate pellet flow. A higher ratio is indicative of a more free flowingpellet.

                  TABLE 1                                                         ______________________________________                                        Elastomer/Plastic Composition                                                 ______________________________________                                                Elastomer.sup.1 : 40% by weight                                               Plastic Polymers                                                              HDPE.sup.2 16% by weight                                                      Polypropylene.sup.3 44% by weight                                     ______________________________________                                         .sup.1 An ethylene propylene copolymer containing 43% ethylene, having a      glass transition temperature of 55° C., and having a mooney            viscosity of 25 (1 + 8 at 127° C.).                                    .sup.2 AB 55100; a 10 melt index HDPE polymer.                                .sup.3 An isotactict polypropylene reactor copolymer of propylene and         ethylene having a crystalline melt temperature of 160° C.         

                  TABLE II                                                        ______________________________________                                                    Wall Friction Angle                                                                         (Degrees)                                           Process     Stainless Steel                                                                             Aged Carbon Steel                                   ______________________________________                                        A Conventional                                                                            22            22                                                  strand cut                                                                    B Underwater cut                                                                          13            18                                                  (skin)                                                                        ______________________________________                                    

                  TABLE III                                                       ______________________________________                                                              Yield                                                           Consolidating Strength Pressure/                                      Process Pressure (psi)                                                                              (psf)    Strength ratio                                 ______________________________________                                        A       386           135      2.49                                           B       272            18      15.1                                           ______________________________________                                    

In preparing the underwater die cut pellets of this invention, the MUPwas operated with an extruder melt temperature of 408° F., an extruderpressure of 1600 psi and cooling water temperature of 105° F. Theextruder output rate was 125 lbs/hr.

It is evident from the above data that pellets made according to theprocess of this invention are more free flowing than strand pelletizedmaterial even where the compositions are identical. FIG. 1A and B areSEM micrograph comparisons of the strand formed pellets (FIG. 1A) andthe underwater cut pellets (FIG. 1B). The rubber phase of thecomposition was extracted with hexane. As can be seen from themicrograph of FIG. 1B, the product produced according to the process ofthis invention has a skin which is substantially all plastic polymerwhile the pelletized strands are essentially of uniform compositionthroughout.

In order to demonstrate that the skin of the pellets of this inventionare substantially all plastic polymer, the pellets were treated withhexane to extract the rubber phase. A comparision of FIGS. 2A and 2Bshow that rubber was extracted from the surface of the conventionalstrand pelletized material, whereas substantially no rubber wasextracted from the sking of the pellets prepared according to theprocess of this invention.

What is claimed is:
 1. A process for preparing a free flowing polymerpellet which comprises:(a) intimately mixing an elastomeric butyl rubberpolymer or a mixture of butyl rubber and ethylene-propylene rubber,ethylene, propylene and a non-conjugated diene, natural rubber,polyisobutylene, halobutyl rubber or styrene butadiene rubber, having aglass transition temperature (Tg) of less than 0° C. with a plasticpolymer having a semicrystalline or crystalline melting point of atleast 10° C. higher than the softening point of the elastomer; saidplastic polymer and elastomeric polymer being insoluble in one anotherin the melt state; (b) extruding the elastomer/plastic polymer mixtureat an extrusion melt temperature above the melting point of the plastic,through a die having a die inlet and a die face said die face being thedie outlet; maintaining a temperature gradient across the die from thedie inlet to the die outlet, said temperature gradient being such thatthe polymer mixture has a melt temperature at the die outlet which isless than the extrusion melt temperature; and (c) forming pellets of theextrudate, thereby forming a pellet having a skin comprising the plasticpolymer.